Wow, I have become somewhat of a board meme.
Anyway, yes, one of your best options to reduce the cultural/linguistic plurality of western Eurasia as drastically as reasonably possible is to have a much more successful Roman Empire that manages to assimilate Germania up to the Vistula-Dniester line and Mesopotamia-Arabia up to the Zagros mountains, and experiences an Imperial China-like evolutionary path.
Celtic languages would get extinct, or quite possibly only survive in Hibernia/Caledonia if Rome never bothers to conquer them up to modern times. Germanic languages would get extinct as well, or quite possibly only survive in Scandinavia if Rome never bothers to conquer it up to modern times. Berber and Semitic languages would be wiped out as well. Egyptian may or may not survive as a regional minority language, due to its cultural prestige and area entrenchment. Hebrew may or may survive as well, depending on how much the Jew community manages to resists assimilation within a successful Roman Empire.
You'd get a huge bilingual, culturally unified imperial blob, perhaps remaining politically unified up to modern times, with occasional periods of disunity, perhaps suffering a prermanent West-East division at some point, with Latin getting universal diffusion in the western half (Western-Central Europe, Maghreb) and Greek in the eastern half (Greece, Egypt, Levant, Anatolia, Mesopotamia, Arabia). The Balkans would probably be a hybrid border area, and most educated people getting at least basic bilingual fluency.
On its northeastern borders, assuming that Rome never manages/bothers to expand into Sarmatia till some kind of Rus-like polity arises and unifies it, you'd get some kind of Gothic-Norse, Slavic, Iranic, or Turkic language, or quite possibly an hybrid of any or all of the above, depending on which culture gets to become regional overlord.
On its southeastern borders, assuming that Rome never manages/bothers to conquer Persia, you'd get the Persian area of course. If Rome eventually does manage to conquer Persia, say in early modern times, Persian could instead become the third language of the Roman Empire instead, surviving thanks to its cultural prestige and area entrenchment, albeit much less widespread than Latin and Greek and limited to regional diffusion.